In an existing cellular system, an array of antennas of a base station is generally arranged horizontally as illustrated by FIG. 1 and FIG. 2. A beam emitted by a transmitter of the base station can only be adjusted in the horizontal direction, and has a fixed dip in the vertical direction, so various beam-forming/pre-coding technologies, etc., operate on the basis of information about a channel in the horizontal direction. In fact, since a radio signal is propagated in three dimensions in space, the performance of the system cannot be optimized with the fixed dip, and adjustment of the beam in the vertical direction is of great significance to reduction in inter-cell interference, and to improvement in performance of the system.
With the development of antenna technologies, active antennas in which each antenna oscillator can be controlled separately has emerged in the industry as illustrated by FIG. 3A and FIG. 3B. With this array of antenna, it becomes possible to adjust the beam dynamically in the vertical direction.
In such a three-dimension array of antennas, beam-forming in both the horizontal direction and the vertical direction can be performed on a signal transmitted by the base station to a user equipment (UE). In order to enable an evolved Node B (eNB) to determine a beam-forming vector in the vertical direction so that the beam in the vertical direction is oriented to the UE to maximum a beam-forming gain, the UE typically needs to feed back CSI in the vertical direction. In a particular implementation, the UE is configured with a plurality of CSI feedback configurations, and different CSI feedback configurations use different vertical beam-forming vectors, and the UE measures and feeds back CSI based upon the configured CSI feedback configurations. Here the CSI generally includes a rank indicator (RI), a pre-coding matrix indicator (PMI), and a channel quality indicator (CQI).
Particular operations are introduced below.
Firstly the base station determines N non-zero power (NZP) channel state information reference signal (CSI-RS) resources, each CSI-RS resource has the same number of ports and the same number of groups of antenna elements for each CSI-RS resource, and each port of each CSI-RS resource corresponds to one group of antenna elements, for example, the first port corresponds to the first column of vertical antennas, the second port corresponds to the second column of vertical antennas, and so on. The base station determines a beam-forming weight vector for each CSI-RS resource, and the beam-forming weight vector can be determined by a vertical angle to be covered by the CSI-RS resource. For each port of the CSI-RS resource, a pilot signal thereof is weighted by the beam-forming vector, and then transmitted from the group of antenna elements corresponding to the port.
As illustrated by FIG. 4, for example, there are 16 antenna elements in total, and four antenna elements in the vertical direction are defined as a group. So each group has four antenna elements and there are four groups in total. Each group of antenna elements are configured to transmit pilot signals of a port of a CSI-RS resource. A pilot signal sn(i) of the i-th port is weighted by a beam-forming weight vector [wn(0) wn(1) wn(2) wn(3)]T, and then transmitted from the i-th group of antennas, i.e., the i-th column of antennas. The subscript n in FIG. 4 distinguishes one CSI-RS resource from another CSI-RS resource. If the base station is configured with three CSI-RS resources, and each CSI-RS resource has a different beam direction, three different sets of beam-forming weight vectors [wn(0) wn(1) wn(2) wn(3)]T needs to be configured, where n=0, 1, 2. The UE can measure respectively on the basis of these three groups of CSI-RS resources, and report CSI measured over a CSI-RS resource with the best channel quality and positional information of that CSI-RS resource among all the configured CSI-RS resources. The base station can obtain a current optimum vertical beam-forming weight vector according to the positional information to perform vertical beam-forming of data.
In a Long Term Evolution (LTE) system, in order to support cooperative multiple-point transmission (CoMP), the concept of CSI process is introduced. Each CSI process can correspond to an NZP CSI-RS configuration and an interference measurement resource (IMR) configuration. The UE measures a channel based upon the NZP CSI-RS and measures interference using the corresponding IMR to obtain and feed back CSI corresponding to each CSI processes. Each LTE UE can be configured with at most three CSI processes to feed back CSI. The UE can feed back CSI over a periodical physical uplink control channel (PUCCH), or over an aperiodic physical uplink shared channel (PUSCH). To feed back CSI periodically, the base station configures a periodical PUCCH resource, and the UE reports corresponding CIS periodically over the configured resource. In order to feed back CSI aperiodically, the base station triggers the UE to feed back CSI via Downlink Control
Information (DCI), and the UE feeds back the CSI over a PUSCH in an uplink sub-frame corresponding to the triggering sub-frame. To feed back CSI aperiodically, the base station can trigger the UE to report CSI corresponding to each CSI process in a certain set of CSI processes, where the UE is pre-configured with the set of CSI processes via higher-layer signaling, and the UE is triggered via DCI. A plurality of sets of CSI processes are configured also to enable the
UE to feed back CSI corresponding to a plurality of CSI-RSes obtained after beam-forming using different vertical beam-forming vectors.
In the existing solution, schemes to report positional information and CSI to the base station respectively in different feedback modes are not supported, schemes of the base station triggering the UE via triggering signaling to feed back CSI in a specified CSI feedback configuration are not supported, either, thus discouraging the base station from more flexible downlink scheduling and transmission.